1. Field of the Invention
The present invention relates to a cylindrical lithium ion secondary battery, and more particularly, to a cylindrical lithium ion secondary battery having a inexpensive center pin while preventing metal leaching in the electrolyte.
2. Description of the Prior Art
Typically, a cylindrical lithium ion secondary battery includes a cylindrical electrode assembly, to which a center pin is fitted, an electrolyte contained in the electrode assembly, a cylindrical case for receiving the electrode assembly and the electrolyte, and a cap assembly assembled to the top opening of the cylindrical case for sealing the cylindrical case.
The electrode assembly includes a positive electrode plate coated with a positive active material, a negative electrode plate coated with a negative active material, and a separator interposed between the positive electrode plate and the negative electrode plate such that an electrical short circuit is prevented and only the movement of lithium ions is allowed. The positive electrode plate, the negative electrode plate and the isolator are wound substantially in a cylindrical form. A positive electrode tab connects the positive electrode plate to the cap assembly, and a negative electrode tab connects the negative electrode plate to the case.
Since such cylindrical lithium ion secondary batteries have a capacity of about 2,000 to 2,800 mAh, they are usually used in notebook computers, digital cameras, camcorders, or the like, which require a large capacity of power. Those secondary batteries are assembled and typically used in the form of a battery pack, in which the batteries are connected in series and/or in parallel according to a required level of voltage or power, and a safety device is provided in the battery pack.
Such a cylindrical lithium ion secondary battery may be fabricated in the following manner.
First, a positive electrode plate which is coated with a positive active material and to which a positive tab is connected, a negative electrode plate which is coated with a negative active material and to which a negative electrode tab is connected, and a separator are laminated and then wound substantially in a cylindrical form, thereby fabricating an electrode assembly. Then, the electrode assembly is introduced into a cylindrical case and a tab is connected to a cap assembly. Electrolyte is then poured into the case, and the cap assembly is assembled to the case, thereby finishing a cylindrical lithium ion secondary battery.
However, the above-mentioned cylindrical lithium ion secondary battery has a problem in that heat is produced by a chemical reaction in the battery when the battery is charged or discharged and such heat may ignite or fracture the battery. Therefore, it becomes necessary to maintain thermal stability for such a cylindrical lithium ion secondary battery by developing a structure capable of dissipating heat produced in the battery to the outside of the battery. Recently, a substantially cylindrical center pin is inserted through the central area of the electrode assembly so that the electrode assembly is not deformed during charging or discharging of the lithium ion secondary battery.
The electrode assembly received in the case is formed with a space at the central area thereof. Such a space formed as a shaft for winding is extracted, wherein the shaft is used when a positive electrode plate and a negative electrode plate are wound with a separator interposed between the positive electrode plate and the negative electrode plate. A cylindrical center pin is inserted into this space. Such a center pin is typically formed by cylindrically rolling up a metal sheet. If the center pin is formed in this manner, a slit having a width is formed in the overlapped portions of the sheet. In order to prevent the formation of such a slit, the center pin may be occasionally fabricated in a one-piece cylindrical form from the beginning.
Stainless steel (SUS) is typically used as a material of such a center pin. Such material prevents the oxidization of the center pin as well as preventing the center pin from leaching in the form of ions in the electrolyte. However, SUS has a disadvantage in that its price is very expensive. Therefore, a need exists for the prevention of leaching in the electrolyte even if a relatively inexpensive metal instead of SUS is used for fabricating such a center pin.